It is known that in a great many hydrocarbon treating processes, such as reforming, cracking, isomerization, hydrotreatment of residues, etc., the hydrocarbon feedstock is converted by being contacted with catalyst particles in a reactor. Most of the treating units are fixed-bed units. (See "Catalytic Reforming", by Donald M. Little, PennWell Books.)
In the course of the catalytic reaction, coke and various hydrocarbon products deposit on the particles and deactivate them. The catalyst particles must then be regenerated by means of a stream of carrier gas which generally contains oxygen to provide for the combustion of the coke and of the other products deposited on the catalyst.
This operation may be performed in situ or ex situ, with the unit shut down.
The advantage of continuous regeneration processes, known as moving-bed processes, is that the production unit need not be shut down and the spent catalyst can be regenerated by withdrawing it downstream of the reactor and, after a regenerating treatment, reinjected into an ad hoc zone connected with the reactor.
However, even with units of the regenerative type, the catalyst is gradually deactivated. It is therefore advisable to replace the spent catalyst periodically with fresh catalyst.
It is during the replacement of this catalyst (hereinafter called the first catalyst) with fresh catalyst (hereinafter referred to as the second catalyst), which may be either of the same type or of a different type from the first catalyst, that difficulties arise in continuous operation when attempting to avoid interruption in production by the hydrocarbon treating unit.
In particular, it is difficult to know just when to stop introducing the second catalyst, so that the first catalyst is definitely completely discharged and so that no excess of second catalyst is introduced upstream of the reactor. This difficulty is due in part to the mixing of the two catalysts which occurs particularly along the walls of the reactor.
In fact, since there is no means to accurately measure the quantities of catalyst to be withdrawn and amount of catalyst to be introduced, it is customary to overcompensate and feed into the reactor a quantity of fresh catalyst that is much greater than what is normally required. This results in a higher cost of the catalyst feed, a corresponding loss of a portion of the fresh catalyst that will be reprocessed as if it had been deactivated, and an increase in the duration of the replacement phase, which imposes an economic penalty on production.
Moreover, when a unit with several parallel reactors is used which reactions are individually connected to a single regenerator and which can be in continuous use, simultaneously, the replacement of the catalyst is very difficult to carry out with the desired synchronicity. For example, the replacement operations may be terminated in one of the reactors while still in progress in the other reactors. The safety margin then becomes prohibitively expensive for the refiner. Also, the mixture of old and new catalyst particles may be very heterogeneous, especially along the reactor walls.